A model system, which consists of the implantation of pieces of bioprosthetic leaflets subcutaneously in rabbits, was developed to study the process of calcification of glutaraldehyde-treated bioprosthetic valves. In this model, significant calcification develops within 3-6 months of implantation. The calcific deposits are morphologically similar to those which occur in bioprosthetic valves implanted in intracardiac positions.